AP Chemistrymediummcq1 pt
A 0.010 M solution of weak acid HA has pH = 4.00. What is Ka for HA?
A.1.0 × 10⁻⁴ M (equal to [H⁺])
B.1.0 × 10⁻² M (the initial HA concentration)
C.1.0 × 10⁻⁶ M
D.2.0 × 10⁻⁶
Explanation
Core Concept
pH 4.00 means [H⁺] = 1.0 × 10⁻⁴ M, and each ionization also forms one A⁻. Substituting into Ka = [H⁺][A⁻]/[HA] gives (1.0 × 10⁻⁴)² ÷ (0.010 - 0.0001) ≈ 1.0 × 10⁻⁶. The x correction barely changes the denominator, confirming the small-x treatment was valid.
Correct Answer
C1.0 × 10⁻⁶ M
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